1. Field of the Invention
The present invention relates to an improvement in polymerizations used to produce polymers having a narrow molecular weight distribution which are used as standard materials for measuring the average molecular weight or molecular weight distribution or other polymers.
2. Description of the Prior Art:
In general, the processability and mechanical properties of synthetic resins and synthetic rubbers produced in the high-polymer chemical industries are determined by the molecular characteristics of the polymers of interest, especially the average molecular weight and the molecular weight distribution of the polymers. Accordingly, it is quite important to be able to accurately measure these properties. It has been known that gel permeation chromatography (hereinafter, referred to as GPC) and ultracentrifugation can be employed for simultaneously measuring the average molecular weight and the molecular weight distribution of a polymer. However, fully suitable standard materials for use in these measurements do not exist, and as a result the development of such standard materials is required. More specifically, a series of standard materials having a quite narrow molecular weight distribution over a wide range of molecular weights from 10.sup.3 -10.sup.7, preferably 10.sup.2 -10.sup.8, is required.
In the past, polystyrenes produced by conventional living anionic polymerization have been used as standard materials. However, the molecular weight distribution of these polymers is insufficiently narrow. Especially in the region of molecular weights greater than 10.sup.6, it is difficult to produce a polymer having a value of M.sub.w /M.sub.n of less than 1.10. M.sub.w /M.sub.n is the ratio of the weight average molecular weight (M.sub.w) to the number average molecular weight (M.sub.n). Consequently, there has been a need to develop a polymerization method for producing polymers having such a narrow molecular weight distribution in the aforementioned molecular weight region. (Hereinafter, these will be referred to as polymers having a narrow molecular weight distribution, i.e., having a value of M.sub.w /M.sub.n of less than 1.10.)
Heretofore, attempts to produce polymers having such a narrow molecular weight distribution have been based on achieving the following conditions:
(1) The rate of initiation of the reaction in the polymerization is much higher than the rate of propagation of the reaction;
(2) Impurities such as water, oxygen, carbon dioxide, and the like which deactivate the active anions are carefully excluded from the polymerization system; and
(3) The monomer and the polymerization initiator are uniformly dissolved in the polymerization system before the initiation of the polymerization reaction. However, for example, in the anionic polymerization of styrene, it is difficult to completely remove the impurities from the monomer as prescribed by the second condition. It is also difficult to attain the third condition because of the extremely high rate of polymerization included in the conventional anionic polymerization reaction. Also, establishing the second condition requires an exceptionally long period of time and a large amount of work in order to suitably purify the solvent for polymerization and the monomer. The results of establishing the third condition are also unsatisfactory. Consequently, there continues to exist a need for a satisfactory polymerization method for producing polymers having a narrow molecular weight distribution.